A conventional derailleur for a bicycle uses a deformable linkage mechanism comprising four members: a base member, two linkage members, and a movable member, so that a control wire is operated to deform the linkage mechanism thereby changing the bicycle speed. In a case of setting the speed change stages, one of two members of the linkage mechanism, which are movable relative to each other, for example, one linkage member and the base member, is provided with a plurality of engaging portions to set the speed change stage and the other is provided with a retainer engageable with one engaging portion to thereby constitute a positioning mechanism.
In the conventional positioning mechanism, however, the engaging portions and retainer constituting the positioning device is provided directly on the members constituting the linkage mechanism, for example, the one linkage member and the base member, or is formed separately therefrom and fixed when the derailleur itself is being assembled. Accordingly, a problem arises in that the members carrying the engaging portions and retainer are complicated in construction and difficult to assemble.
Also, since the positioning mechanism is non-removably incorporated in the linkage mechanism, in order to provide users with a selection between a derailleur which, at the factory, has or has not been previously provided with a positioning mechanism, two different kinds of derailleurs must be produced. Also, when the multistage sprocket assembly has a different sprocket constitution, a derailleur having a positioning mechanism corresponding to the different sprocket constitution must be used.